Today, highly caffeinated drinks are more popular than ever, especially among teens. But one leading energy drink, Monster, is currently under investigation by the Food and Drug Administration for a possible link to five deaths.

The FDA is looking into reports of five deaths and one non-fatal heart attack that may have been caused by Monster, the leading energy drink in the U.S. with almost 39 percent of the market share. Monster is also being sued by the parents of 14-year-old Anais Fournier, who died after drinking two cans over the course of a 24-hour period.

Monster is defending itself against the allegations, arguing that it's "unaware of any fatality anywhere that has been caused by its drinks." Further, the company doesn't believe that its beverage is "in any way responsible" for Fournier's death.

Fournier's family says the drink complicated a preexisting heart condition and that she died from "cardiac arrhythmia due to caffeine toxicity." The drink contains 240 milligrams of caffeine, which is seven times the amount found in a 12-ounce cola.

The FDA, meanwhile, is investigating five death reports from 2009 through June of this year. According to the FDA, they routinely investigate all claims of death or injury, but this doesn't necessarily indicate a problem with the popular drink.